Self-tapping screws are commonly of two types, one type including a portion of the screw which cuts away material from the workpiece to produce a thread, and the other type forms a thread in a bore of a workpiece by causing the metal or other material of the workpiece to be deformed or flow as the thread-forming screw is forced into the bore. The latter type of self-tapping screw or thread-forming screws are commonly preferred over the thread-cutting screws because a thread-forming screw work-hardens the workpiece as the thread-forming screw is forced into the bore.
Such thread-forming screws commonly include a tapered leading end which is at least partially threaded. There are two common methods for use in forming a thread-forming screw. In one such method threads are rolled on a screw blank including a tapered end portion, using conventional flat thread rolling dies. When flat thread rolling dies are employed, the threads on the tapered end portion of the screw are not fully formed and do not have sharp crests. Since these leading threads are not sharp, they will not readily deform the metal of the workpiece, and when the thread-forming screws are forced into the workpiece, a substantial torque must be applied on the screw to cause it to form the desired thread.
In an alternative method for making a thread-forming screw, the dies used to roll the threads include tapered portions conforming to the taper of the screw blank and particularly constructed to form the threads on the tapered leading end of the thread-forming screw. Screws formed in this manner are preferred because the threads on the tapered portion of the screw blank will be provided with sharp crested threads. Accordingly, these thread-forming screws can be more efficiently forced into the bore in a workpiece and substantially less torque is required. On the other hand, screws formed in this manner and through the use of dies that include tapered portions are substantially more expensive, in part, because the dies used for rolling the threads are substantially more expensive than conventional flat thread rolling dies.